1. Field of the Invention
The present invention relates generally to a C-type frame for an injection molding machine. More particularly, the present invention relates to a frame having a substantially C-shaped contour as seen from the side wherein the frame is employable for die tightening units mounted for an injection molding machine for injecting a molten synthetic resin.
2. Background Art
Almost all of the C-type frames have been hitherto produced using steel plates each having a heavy thickness. Specifically, each C-type frame is produced by cutting a steel plate to dimensions of an opposing pair of side frames, upper frames and a front frame extending between both the side frames and then welding them to each other to build an integral structure corresponding to the C-type frame.
Because of the geometrical configuration of the conventional C-type frame as mentioned above, the C-type frame is produced with a large number of manhours consumed therefor. In spite of the fact that it is simple in structure, it is produced at low productivity, resulting in it being unavoidably produced at an expensive cost.
To obviate the foregoing problems, some manufacturers have employed a casting process for producing a C-type frame at improved productivity. However, when the C-type frame is produced by casting, its tensile strength, compressive strength and elastic modules are inferior to those of a C-type frame produced using steel plates having a heavy thickness. Thus, when the manufacturer tries to produce a C-type frame with a reduced quantity of deflection, it is inevitably necessary that each frame member constituting the C-type frame be designed to have a thickness larger than that of a frame member made of a steel plate.
In addition, when the C-type frame is produced with enlarged dimensions, its weight and whole size increase. However, this is contradictory to the latest requirement for designing an injection molding machine with a light weight and small dimensions. For this reason, practical application of a C-type frame produced by employing a casting process is limited only to some of the die tightening units.
In the circumstances as mentioned above, to improve productivity, the manufacturer has tried to design a C-type frame in such a manner that an upper frame is projected forward of the front upper part of a cast main frame, and a base board constituting the bottom of the main frame is integrated with the main frame. In spite of the foregoing designing, however, a tensile strength, a compressive strength and an elastic modules of the cast C-type frame are inferior to those of the C-type frame produced using steel plates having a heavy thickness with the result that the C-type frame is largely deformed when it is loaded with certain exterior force, and especially, the open distance between the main frame and the base board is liable to enlarge.
A quantity of deformation of the C-type frame during the loading has a significant effect on reliable tightening of an injection molding die. If it exceeds a specified allowable quantity, the injection molding die is incompletely opened, resulting in the dimensional accuracy of a molded product being degraded. Although the allowable quality differs depending on a size of each die tightening unit, in case that an injection molding machine is designed with smaller dimensions, it is necessary that a movable board be in parallel with a stationary board within the range of 0.1 mm or less.
It is obvious that the quantity of deformation of the C-type frame can be reduced by designing each frame member constituting the main frame to have a large thickness and an increased weight. However, the designing of the C-type frame in that way is not preferable because an injection molding machine is produced at an expensive cost, and moreover, the weight of the injection molding machine increases.
On the other hand, enlargement of the open distance between the main frame and the base board can be prevented or suppressed by enlarging a diameter of each of bolts extending therebetween or increasing the number of bolts. In spite of the measure taken for the foregoing purpose, however, the present status is such that all the aforementioned problems are left still unsolved.